1. Field of the Invention
The present invention relates to a driving apparatus for a stepping motor.
2. Prior Art
Some vehicle air conditioners use a PM (permanent Magnet) type stepping motor in an actuator driving a door for switching blowing-out ports in a duct, or the like. To drive the stepping motor, a unipolar driving system (a first prior art) and a bipolar driving system (a second prior art) have been used.
(The First Prior Art)
As shown in FIGS. 10A and 10B, a stepping motor 10 of a unipolar driving system has a rotor 11 and first to fourth exciting coils L1-L4 which apply a rotating magnetic field to the rotor 11. The respective ends of the first to the fourth exciting coils L1-L4 are connected to the plus terminal of a power supply V via an external terminal P0 of the motor 10. The respective other ends of the first to the fourth exciting coils L1-L4 are connected to a driving circuit 12 via external terminals P1-P4, respectively.
The driving circuit 12 includes four npn-type bipolar transistors Tr1-Tr4 and four flywheel diodes D1-D4. Each of the diodes D1-D4 is connected between a collector and an emitter of one of the transistors Tr1-Tr4. The collectors of the transistors Tr1-Tr4 are connected to the other ends of the first to the fourth exciting coils L1-L4 via the external terminals P1-P4, respectively, and the emitters thereof are connected to the ground GND.
Control signals .O slashed.1-.O slashed.4 from a control circuit 13 are input to the bases of the transistors Tr1-Tr4, respectively, and the transistors Tr1-Tr4 are selectively turned on and off by the control signals .O slashed.1-.O slashed.4.
Thus, in the driving circuit 12 of the stepping motor 10, the transistors Tr1-Tr4 are sequentially turned on and off on the basis of the control signals .O slashed.1-.O slashed.4 from the control circuit 13 and the first to the fourth exciting coils L1-L4 are sequentially excited. The sequential excitations of the first to the fourth exciting coils L1-L4 causes the rotating magnetic field to be generated and the rotor 11 to be rotated.
(The Second Prior Art)
As shown in FIGS. 11A and 11B, a stepping motor 20 of a bipolar driving system has a rotor 21 and first and second exciting coils L11 and L12 that apply a rotating magnetic field to the rotor 21. Both ends of the first exciting coil L11 are connected via external terminals P11 and P12, respectively, to a first driving portion 22a of a driving circuit 22. Both ends of the second exciting coil L12 are connected via external terminals P13 and P14, respectively, to a second driving portion 22b of the driving circuit 22.
The first driving portion 22a includes a bridge circuit 23a including four npn-type bipolar transistors Tr11-Tr14, and four flywheel diodes D11-D14 each connected between a collector and an emitter of each of the transistors Tr11-Tr14. A power supply V is supplied between nodes N1 and N2, where the node N1 is located between the collectors of the transistors Tr11 and Tr12, and the node N2 is located between the emitters of the transistors Tr13 and Tr14. A node N3, between the emitter of the transistor Tr11 and the collector of the transistor Tr13, is connected via the external terminal P11 to the one end of the first exciting coil L11. A node N4 existing between the emitter of the transistor Tr12 and the collector of the transistor Tr14 is connected via the external terminal P12 to the other end of the first exciting coil L11.
The second driving portion 22b includes a bridge circuit 23b including four npn-type bipolar transistors Tr15-Tr18, and four flywheel diodes D15-D18 each connected between a collector and an emitter of each of the transistors Tr15-Tr18. The power supply V is supplied between nodes N5 and N6, where the node N5 is located between the collectors of the transistors Tr15 and Tr16, and the node N6 is located between the emitters of the transistors Tr17 and Tr18. A node N7 existing between the emitter of the transistor Tr15 and the collector of the transistor Tr17 is connected via the external terminal P13 to the one end of the second exciting coil L12. A node N8 existing between the emitter of the transistor Tr16 and the collector of the transistor Tr18 is connected via the external terminal P14 to the other end of the second exciting coil L12.
Control signals .O slashed.11-.O slashed.18 from a control circuit 24 are input to the bases of the transistors Tr11-Tr18, respectively, and the transistors Tr11-Tr18 are selectively turned on and off on the basis of the control signals .O slashed.11-.O slashed.18.
More specifically, as shown in FIG. 12, the control circuit 24 first turns on only the transistors Tr11 and Tr14 (Step 1). This causes an exciting current to flow through the first exciting coil L11 in the direction of an arrow A1 and a magnetic field is generated on the basis of the exciting current. Next, the control circuit 24 turns on only the transistors Tr15 and Tr18 (Step 2). This causes an exciting current to flow through the second exciting coil L12 in the direction of an arrow A2 and a magnetic field is generated on the basis of the exciting current. Subsequently, the control circuit 24 turns on only the transistors Tr12 and Tr13 (Step 3). This causes an exciting current to flow through the first exciting coil L11 in the direction of an arrow A3 and a magnetic field is generated on the basis of the exciting current. Next, the control circuit 24 turns on only the transistors Tr16 and Tr17 (Step 4). This causes an exciting current to flow through the second exciting coil L12 in the direction of an arrow A4 and a magnetic field is generated on the basis of the exciting current.
Thus, in the driving circuit 22 of the stepping motor 20, the transistors Tr11-Tr18 are turned on and off in the order of Step 1 to Step 4 on the basis of the control signals .O slashed.11-.O slashed.18 from the control circuit 24 and the first and the second exciting coils L11 and L12 are excited in a predetermined timing and polarity. These excitations of the first and the second exciting coils L11 and L12 cause the rotating magnetic field to be generated and the rotor 21 to be rotated.
The first prior art motor 10 described above is more advantageous than the second prior art motor 20 in that the driving circuit 12 can easily be formed with fewer transistors. However, when the motor 10 has the same dimension as the motor 20, the motor 10 requires much more exciting current than the motor 20, when both of the motors 10 and 20 provide the same outputs. Therefore, the motor 10 generates a large amount of heat and is inefficient. Conversely, although the motor 20 generates less heat and is more efficient than the motor 10, the motor 20 has a problem in that the driving circuit 22 must be formed with many transistors and is expensive.
There is a demand for a stepping motor having both features of a high efficiency and a low cost. Accordingly, the driving circuit 22 for the motor 20 (bipolar driving system) having a high efficiency should be simplified in circuit structure.